1. Field of the Invention
The present invention relates generally to a fuel control system for a cylinder injection type internal combustion engine for a motor vehicle in which fuel is injected directly into engine cylinders. More specifically, the present invention is concerned with a fuel control system for the cylinder injection type internal combustion engine which system is so designed as to positively prevent fuel injectors for a plurality of engine cylinders from being actuated simultaneously in order to allow the fuel injectors to be driven by using a power supply source of a low capacity.
2. Description of Related Art
In general, in the internal combustion engine for the motor vehicle and others, an injector for fuel injection is installed in an intake manifold of an intake pipe of the engine so that the fuel as injected is charged into engine cylinders together with the intake air.
For having better understanding of the principle underlying the invention, technical background thereof will be described in some detail. FIG. 4 is a schematic diagram showing a conventional fuel control system for an internal combustion engine in which a fuel injector is mounted on an intake pipe.
Referring to FIG. 4, an internal combustion engine 1 for a motor vehicle includes a plurality of cylinders. For simplification, only one of the cylinders is shown representatively in FIG. 4. The engine 1 is equipped with an intake pipe or manifold la which serves for charging a mixture of intake air and fuel into the engine 1. An exhaust gas resulting from the combustion of the air-fuel mixture within the engine 1 is discharged through an exhaust pipe 1b. A crank shaft 1c is driven rotationally by the engine 1. Cooling water 1d is fed for cooling the engine 1.
An air flow sensor 2 is installed at an inlet port of the intake pipe 1a for measuring the amount of air fed to the engine 1, whereby intake air flow information A is outputted as a detection signal by the air flow sensor 2. Furthermore, mounted within the intake pipe 1a is a throttle valve 3 which is operatively coupled to an accelerator pedal (not shown) operated by a driver of the motor vehicle, for regulating the air flow supplied to the engine 1 in dependence on the depression stroke of the accelerator pedal.
For the purpose of detecting angular positions of the throttle valve 3, i.e., the throttle opening degree .theta., a throttle position sensor 4 is provided in association with the throttle valve 3. Provided in association with the crank shaft 1c is a crank angle sensor 5 for detecting the rotation speed of the crank shaft 1c to thereby output a pulse signal RE having a variable repetition frequency indicative of the rotation speed of the crank shaft 1c. Thus, it is possible to derive from the pulse signal RE information concerning the rotation speed (rpm) of the engine 1 as well as information concerning the angular position of the crank shaft 1c.
Temperature Tw of cooling water 1d is detected by a water temperature sensor 6 which thus serves as a means for detecting the warmed-up state of the engine 1.
An O.sub.2 -sensor 7 is provided in association with the exhaust pipe 1b for detecting an oxygen concentration Do of the exhaust gas discharged from the engine 1 into the exhaust pipe 1b.
For the purpose of controlling the operations of the internal combustion engine system described above, a control unit 8 is provided, which may be implemented in the form of a microprocessor or microcomputer. The detection signals A, .theta., RE, Tw and Do outputted from the various sensors 2, 4, 5, 6 and 7 mentioned above are supplied as the input information to the control unit 8 which in turn outputs control signals B, J and Q for various devices or units in response to the input information to thereby perform various sequence controls inclusive of the fuel injection control and the ignition timing control for each of the cylinders of the engine 1, as will be described hereinafter.
More specifically, the control unit 8 makes decision as to the operation state of the engine 1 on the basis of the various sensor output information A, .theta., RE, Tw and Do mentioned above to arithmetically determine various control quantities in dependence on the detected operation state of the engine 1 and output various operation control signals B, J and Q for allowing the engine 1 to be driven with desired air-fuel ratio and ignition timing.
To this end, a spark plug 9 is mounted within each of the cylinders of the engine 1, being exposed to the combustion chamber defined within the cylinder, wherein the firing of the spark plug 9 is controlled by the ignition timing control signal Q outputted from the control unit 8.
As can be seen in FIG. 4, provided in parallel to the intake pipe 1a across the throttle valve 3 is a bypass passage BP in which an air bypass valve 10 is installed for selectively opening or closing the bypass passage BP. Thus, it is possible to control the air flow bypassing the throttle valve 3 by controlling the air bypass valve 10.
More specifically, operation of the air bypass valve 10 is controlled by the bypass control signal B outputted from the control unit 8, whereby the rate of air flow bypassing the throttle valve 3 can be regulated. In this way, engine rotation speed control in the idling operation state of the engine in which the throttle valve 3 is fully closed as well as engine torque control in the running state of the motor vehicle can be realized.
Referring continuously to FIG. 4, a fuel injector 11 is mounted within the intake manifold located downstream of the intake pipe 1a. Operation of the fuel injector 11 is controlled by the fuel injection control signal J outputted from the control unit 8, whereby the quantity of fuel supplied to the engine 1 is controlled correspondingly.
The fuel is stored in a fuel tank 12 and taken out by means of a fuel pump 13.
Disposed between the fuel pump 13 and the fuel injector 11 is a fuel pressure regulator 14 which serves for control a fuel pressure Pb at the intake port of the fuel injector 11 in dependence on the intake pipe pressure (pressure prevailing within the intake pipe) Pa.
The intake pipe pressure Pa can be measured through the medium of a pipe communicated to the intake pipe 1a, while the fuel pressure Pb is so controlled that the differential pressure (Pb-Pa) is maintained to be constant (at ca. 3 atm.) with reference to the intake pipe pressure Pa by means of an elastic valve element (not shown) incorporated in the fuel pressure regulator 14 so as to operate in response to the intake pipe pressure Pa.
Thus, in the system in which the fuel is fed to the engine cylinders from the fuel injector 11 disposed in the intake pipe 1a, as shown in FIG. 4, the fuel pressure Pb at the fuel injector 11 is so controlled as to assume a level corresponding to the intake pipe pressure Pa plus a predetermined pressure (ordinarily a ca. 3 atm.) by means of the fuel pressure regulator 14.
Now, the principle of the fuel injection in the system described above will be elucidated.
When the fuel injector 11 is driven in response to the fuel injection control signal J issued by the control unit 8, a fuel supply passage is opened or established between the fuel tank 12 (fuel supply source) and the intake port (fuel injection port) of the fuel injector 11, whereby the fuel is injected into the intake pipe 1a under the differential pressure (ca. 3 atm.) between the fuel pressure Pb of the fuel supply source and the intake pipe pressure Pa acting on the fuel injection port of the injector 11.
In the fuel injection system in which the fuel is injected by the injector mounted externally of the engine cylinder, as shown in FIG. 4, a part of the fuel injected from the injector 11 will adhere to inner walls of the intake pipe 1a and surfaces of the intake valves of the engine 1, which means that loss occurs in the amount of the fuel to be injected into the engine cylinders. In this conjunction, it is noted that such fuel deposition has to be taken into consideration particularly when the engine is operating at a low temperature (such as engine starting operation) and when the engine is in a transient operation state where the amount of fuel to be supplied to the engine has to be changed at a relatively high speed. In the above-mentioned cases, the quantity of fuel to be supplied to the engine tends to increase, exerting unwanted influence to the content of harmful components carried by the exhaust gas.
Under the circumstances, there has been developed a cylinder injection type engine equipped with a fuel control system which is designed for injecting the fuel directly into the engine cylinders by controlling the fuel injection timings for a plurality of fuel injectors, as is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 237854/1992 (JP-A-4-237854).
When such direct fuel injection control system is used for conventional gasoline engines, there can be realized advantageous effects mentioned below.
(1) Reduction of the content of harmful gases contained in the exhaust gas
Since the fuel is directly injected into the combustion chamber in the vicinity of the spark plug 9 (see FIG. 4), the air-fuel ratio may be lowered so that the air-fuel mixture becomes lean without taking into consideration the delay involved in the transportation of the fuel, whereby contents of harmful HC (hydro carbon) gas and CO (carbon monoxide) gas carried by the exhaust gas can be reduced.
(2) Reduction of fuel cost
Because the fuel is injected immediately before the ignition timing, there is formed a mass of combustible fuel mixture around the spark plug 9 at the time of ignition, making nonuniform the distribution of the gas mixture containing the fuel. Thus, the fuel-air mixture undergoes a so-called stratified combustion. Consequently, the air-fuel ratio in appearance between the amount of air and that of the fuel charged into the engine cylinder can be significantly decreased so that the air-fuel ratio of the air-fuel mixture becomes correspondingly lean.
Besides, owing to realization of the stratified combustion, combustion of the air-fuel mixture is scarcely affected adversely even when the exhaust gas is recirculated with an increased ratio (i.e., notwithstanding of increased exhaust gas recirculation or EGR in abbreviation). By virtue of this feature, the intake air quantity can be increased with pumping loss being reduced, which can enhance the fuel-cost performance of the internal combustion engine system.
(3) Increased output power of the engine
Since the air-fuel mixture is substantially concentrated around the spark plug 9, the amount of end gas (air-fuel mixture gas in the regions located remotely from the spark plug 9) decreases favorably to the stratified combustion mentioned above, whereby the anti-knocking performance of the engine can be enhanced with the compression ratio in the engine 1 being increased.
Furthermore, because the fuel is converted into gas or gasified within the cylinder, the intake air is deprived of heat as vaporization heat. Consequently, the density of the intake air can be increased, which is effective for enhancing the volumetric efficiency.
(4) Enhancement of drivability
By virtue of the direct fuel injection into the cylinder, the time taken for generation of output torque by the engine 1 from the fuel injection through the fuel combustion is shorter when compared with the engine system shown in FIG. 4, whereby the internal combustion engine system capable of responding to the demand of the driver with high speed can be realized.
Since the advantageous effects such as mentioned above are expected to be obtained, the cylinder injection type fuel control system tends to be very attractive as the ideal fuel control system for the internal combustion engine.
In this conjunction, it is however noted that in the cylinder injection type fuel control system, the fuel pressure applied to the injector should be higher than the pressure within the cylinder into which the fuel is injected and is usually set at a high pressure on the order of several ten atm.
However, the pressure within the cylinder into which the fuel is directly injected (hereinafter this pressure will also be referred to as the intra-cylinder pressure) changes significantly in dependence on the four-cycle engine strokes (suction, compression, combustion and exhaust strokes) as well as positions of the crank shaft 1c and the piston, which in turns means that differential pressure between the fuel pressure and the intra-cylinder pressure varies correspondingly. As a result, the relation between the injector actuating or driving time point and the amount of the fuel injected actually from the fuel injector changes under the influence of the intra-cylinder pressure prevailing at the time point of fuel injection.
In the following, a fuel control system for a cylinder injection type internal combustion engine will be described in more concrete.
FIG. 5 is a schematic diagram showing generally a structure of a conventional fuel control system for a cylinder injection type internal combustion engine. In FIG. 5, components or parts same as or equivalent to those mentioned hereinbefore are denoted by like reference characters as those used in FIG. 4 and repeated description thereof is omitted.
Referring to FIG. 5, a control unit 8A fetches a cylinder identifying signal SG outputted from a cylinder identifying sensor (not shown), and a fuel injector denoted by reference numeral 11A is installed directly in the combustion chamber of the cylinder of the engine 1. For controlling the direct fuel injection, the control unit 8A of the fuel control system now under consideration is so designed as to arithmetically determine the amount of the fuel for injection and the injection timing for thereby controlling operation of the fuel injector 11A in accordance with the result of the arithmetic operation at least in one of the suction stroke and the compression stroke.
A fuel pump 13A and a fuel pressure regulator 14A constituting parts of the fuel supply system differ from those of the system shown in FIG. 4 in that the fuel pump 13A is designed for supplying or feeding the fuel at a high pressure while the fuel pressure regulator 14A is so designed as to control the fuel pressure Pb applied to the direct fuel injector 11A to be constant at a high pressure (ca. several ten atm.) with reference to the atmospheric pressure PA.
Further, an injector driver 15 is provided for driving the injectors 11A with high output power and designed so as to output a power-amplified injection control signal K in response to the fuel injection control signal J issued by issued by the control unit 8A.
Additionally, a cylinder identifying sensor 16 is provided for identifying the cylinder to be controlled for thereby outputting the cylinder identifying signal SG to the control unit 8A in response to the operation of the intake valve of the engine 1.
In the fuel control system for the cylinder injection type engine such as shown in FIG. 5, the injector driver 15 is provided separately from the control unit 8A in order to drive the fuel injector 11A with very high power by overcoming the intra-cylinder pressure. Besides, because of the necessity for controlling individually the fuel injectors 11A on a cylinder-by-cylinder basis, the cylinder identifying sensor 16 is provided for identifying the cylinder to be controlled.
As will be understood from the above, the direct fuel injector 11A is supplied with fuel at high pressure. Consequently, a fuel supply controlling movable member (e.g. plunger not shown) incorporated in the direct fuel injector 11A has to be implemented in such structure as to be capable of feeding the fuel by overcoming the high fuel pressure Pb applied to the fuel injector 11A.
For these reasons, the spring constant for the spring for actuating the plunger (not shown) incorporated in the direct fuel injector 11A is set at a high value. In this conjunction, it is noted that in the direct fuel injector 11A, the plunger is actuated under the electromagnetic force generated by a coil upon electric energization thereof in response to an electric injection control signal K. Thus, it goes without saying that the coil or solenoid for actuating the plunger has to be so designed as to be capable of generating the electromagnetic force of large magnitude sufficient for overcoming the preset high spring constant of the spring provided in association with the plunger.
In order to meet the requirement mentioned above, the solenoid or coil of the plunger of the direct fuel injector 11A is designed to exhibit low impedance, as a result of which a large current is required for energization of the coil for the plunger. Thus, great difficulty will be encountered in the attempt for integrally incorporating the injector driver 15 in the control unit 8A.
Such being the circumstances, in most of the fuel control system for the cylinder injection type internal combustion engine, the injector driver 15 dedicated for driving the fuel injector 11A is provided separately from the control unit BA, as can be seen in FIG. 5.
Next referring to FIG. 6, the principle of operation of the injector driver 15 shown in FIG. 5 will be described.
In the internal combustion engine 1 of the cylinder injection type, driving force of a great magnitude is required for operating the direct fuel injector 11A, as mentioned above. Besides, fuel injection must be performed only during the suction stroke and the compression stroke. For these reasons, the direct fuel injector 11A is usually driven by resorting to overexcitation technique.
FIG. 6 is a waveform diagram illustrating the injection control signal K and a driving current I flowing through the coil or solenoid of the fuel injector 11A upon driving thereof with overexcitation. Referring to FIG. 6, the driving operation of the fuel injector 11A is started with overexcitation at a time point TON and terminated at a time point TOFF2 with the overexcitation driving period ending at a time point TOFF1. Upon start of the fuel injection, the plunger of the injector is driven with the overexcitation current I1 and held with a holding current I2.
More specifically, when the injection control signal K of high power is outputted from the injector driver 15 in response to the fuel injection control signal J issued by the control unit 8A at the time point TON, the driving current I rises up instantaneously to the high level I1. Thus, the plunger of the fuel injector 11A is speedily actuated immediately after the time point TON.
When the direct fuel injector 11A is opened due to the actuation of the plunger, the driving current I assumes the low level I2 at the time point TOFF1, whereon the holding current I2 is sustained up to the time point TOFF2 at which the driving current I of the fuel injector 11A is interrupted in response to clearing of the injection control signal K delivered from the injector driver 15. Thus, the direct fuel injector 11A is closed substantially at the time point TOFF2.
Next, referring to FIG. 7, description will be turned to a typical structure of the injector driver 15. Incidentally, FIG. 7 is a block diagram illustrating an internal structure of the injector driver 15 on the assumption, only by way of example, that the internal combustion engine of concern includes four cylinders #1, . . . , #4 and that injection control signals K1, . . . , K4 for the cylinders #1, . . . , #4 are sequentially outputted in response to fuel injection control signals J1, . . . , J4 for the cylinders #1, . . . , #4, respectively.
Referring to FIG. 7, a high-voltage generating circuit 401 serves as a power supply for generating a high voltage VH, starting from a battery voltage VB, to thereby output a large or high-level current I1 (see FIG. 6) during a period from the time point TON to the time point TOFF1 (this period will be hereinafter referred to also as the overexcitation period).
Ordinarily, in the motor vehicle, the battery voltage VB is on the order of 14 volts. The high-voltage generating circuit 401 is so designed as to generate the high voltage VH boosted up to several ten volts.
The high-voltage generating circuit 401 should ideally be implemented with a large capacity sufficient for driving the four injectors 11A simultaneously. Alternatively, a same number of high-voltage generating circuits as the direct fuel injectors 11A should be provided separately for the these injectors 11A, respectively. However, in view of the fact that implementation of the high-voltage generating circuit 401 on a small scale is preferable for the practical purpose and that for the direct fuel injection, the injection timing is restricted with the fuel injection being performed basically sequentially on a cylinder-by-cylinder basis, the high-voltage generating circuit 401 is usually realized on a circuit scale capable of supplying a large current enough for driving only one direct fuel injector 11A with the overexcitation current.
The injector driver 15 further includes waveform shaping circuits 402 to 405 provided for the individual cylinders, respectively, for shaping the fuel injection control signals J1, . . . , J4 supplied from the control unit 8A on a cylinder-by-cylinder basis to thereby output waveform-shaped signals J1', . . . , J4', respectively. Additionally, the injector driver 15 includes an overexcitation signal generating circuit 406 for generating an overexcitation signal DH on the basis of the high voltage VH and the waveform-shaped signals J1', . . . , J4'. Furthermore, driving circuits 407, . . . , 410 are provided in association with the engine cylinders, respectively, for generating injection control signals K1, . . . , K4 on the basis of the overexcitation signal DH and the waveform-shaped signals 31', . . . , J4', respectively.
Referring to FIG. 7, the overexcitation signal generating circuit 406 is supplied with the high voltage VH to output the overexcitation signal DH to the driving circuits 407, . . . , 410 for driving the associated direct fuel injector 11A, respectively, with overexcitation until the driving current I of the direct fuel injector 11A rises up to the predetermined high level I1.
The driving circuits 407, . . . , 410 fetch the waveform-shaped signals J1', . . . , J4', respectively, and the overexcitation signal DH, to thereby output the injection control signals K1, . . . , K4 to the fuel injectors 11A for the cylinders, respectively, to thereby cause the driving current I to flow through the fuel injector, as shown in FIG. 6.
Now, it is supposed, by way of example, that two direct fuel injectors 11A are to be simultaneously activated by means of the injector driver 15 shown in FIG. 7 with the overexcitation current. In that case, however, since it is presumed that the high-voltage generating circuit 401 is implemented with the current supply capacity capable of driving only one injector with the overexcitation current, no more than one fuel injector 11A can be driven with the overexcitation current at one time, incurring thus a delay in the driving of the other injector (delay in the fuel injection timing) or failure in driving the other injector in the worst case.
Next referring to FIGS. 8A to 8D, the simultaneous overexcitation will be described in detail.
FIGS. 8A to 8D are timing charts for illustrating, by way of example, fuel injection timing controls when the operation mode of the engine 1 is changed over from the compression stroke injection mode to the suction stroke injection mode. Such change-over of the fuel injection timing control mode is automatically effectuated by the control unit 8A when the operation mode of the engine is changed over to the accelerating operation mode.
More specifically, in the normal engine operation mode, the fuel injection is carried out in the compression stroke with a view to reducing the fuel cost by making the air-fuel mixture lean. However, in the accelerating operation mode of the engine, the fuel injection is carried out in the suction stroke in order to enrich the air-fuel mixture for thereby increasing the output power of the engine.
In that case, in the transient state intervening between the compression stroke mode and the suction stroke mode, there may arise the possibility that coincidence or overlap occurs between the current fuel injection timing and the preceding fuel injection timing.
Referring to FIG. 8A, the cylinder identifying signal SG outputted from the cylinder identifying sensor 16 indicates a specific cylinder #1 which is in the compression stroke (#1 TOP). The pulse signal RE outputted from the crank angle sensor 5 represents crank angle reference position B75.degree. (75.degree. before the top dead center) and B5.degree. (5.degree. before the top dead center) for each of the engine cylinders #1, . . . , #4.
The control unit 8A identifies the fuel injector 11A to be controlled on the basis of the information contained in the cylinder identifying signal SG and predicts or estimates the rotational positions of the crank shaft 1c for each of the engine cylinders, respectively, on the basis of the frequency or period of the pulse signal RE to thereby perform the control such as the fuel injection timing control.
In the conventional four-cylinder engine, the pulse signal RE includes pulses inverting at the crank angles of B5.degree. and B75.degree. periodically with an interval of 180.degree. CA (in terms of the crank angle) for each of the engine cylinders. The control unit 8A fetches various information including the intake air flow information A in the engine 1 at every reference position B5.degree. to decide the engine operation state for thereby arithmetically determining the control quantities which are outputted in the form of the various control signals B, J and Q mentioned previously.
FIGS. 8B to 8D are timing charts showing the fuel injection control signals J1, . . . , J4 for the individual cylinders (#1, . . . , #4), respectively. The fuel injection is performed every time the fuel injection control signals J1, . . . , J4 assume a high level (H-level).
More specifically, FIG. 8B shows waveforms for the fuel injections in the compression stroke injection mode in which the fuel injection is performed immediately before the compression stroke (TOP) for each of the engine cylinders. On the other hand, FIG. 8C shows waveforms for the fuel injections in the suction stroke injection mode. As can be seen from FIG. 8C, the fuel injection is performed in the suction stroke which precedes to the compression stroke by 180.degree. CA in each of the engine cylinders.
It can further be seen that in the fuel injection in the compression stroke in which the air-fuel mixture is lean (FIG. 8A), the pulse width of the fuel injection control signals J1, . . . , J4 is set short, while in the fuel injection in the suction stroke in which the air-fuel mixture is rich, the pulse width of the fuel injection control signals J1, . . . , J4 is set long.
Finally, FIG. 8D shows waveform of the injection pulses in the transient state of the engine which intervenes between the fuel injection in the compression stroke (FIG. 8B) and the fuel injection in the suction stroke (FIG. 8C). It can be seen from FIG. 8D that the fuel injection is performed sequentially for the cylinders #1 and #3 in the compression stroke and thereafter the fuel injection is performed for the cylinder #4 in the suction stroke.
Consequently, the injection timing for the cylinder #4 advances or shifts in the leading direction about one stroke (180.degree. CA) when compared with the injection in the compression stroke (refer to the pulse depicted by a broken line). As a result of this, the injection pulse waveform representing the injection in the compression stroke for the cylinder #3 overlaps with the injection pulse waveform representing the fuel injection in the suction stroke for the cylinder #4.
For the reason mentioned above, the output power of the injector driver 15 mentioned previously may become too insufficient to drive the fuel injector 11A for one of the cylinders #3 and #4 or both of them satisfactorily, which will of course incur lowering in the output power or torque of the engine 1 due to loss of the combustion.
The problem mentioned above may be coped with by increasing the current supply capability or capacity of the high-voltage generating circuit 401 to such a level that the overexcitation driving of plural fuel injectors can be realized. However, in that case, the circuit scale of the injector driver 15 will necessarily increase, unfavorably for practical applications.
As will now be understood from the foregoing description, in the fuel control system for the cylinder injection type engine known heretofore, the high-voltage generating circuit 401 (see FIG. 7) incorporated in the injector driver 15 is of such a current supply capacity which is sufficient for overexciting only one fuel injector 11A. Consequently, when the simultaneous fuel injection is to be performed for a plurality (e.g. two) of the engine cylinders, the overexcitation fuel injection control can be performed for no more than one injector 11A, incurring a delay in the start timing for the fuel injection control and loss of the fuel injection in the worst case, giving rise to problems.
For solving the problems mentioned above, it may be conceived to increase the current supply capability or capacity of the high-voltage generating circuit 401 to a level capable of driving simultaneously a plurality of fuel injectors. In that case, however, the circuit scale of the high-voltage generating circuit 401 increases undesirably in view of the manufacturing cost, unfavorably from the economical and practical viewpoint.
Furthermore, overlap of the fuel injector actuation timings between the engine cylinders may be avoided by such control processing that upon every actuation timing for the direct fuel injectors 11A of the individual engine cylinders, proximity of the actuation timing for the fuel injectors 11A of the other cylinders is checked for all the fuel injectors 11A. In that case, the burden or overhead imposed on the control unit BA for executing the above-mentioned processing upon every fuel injection will increase intolerably.